The invention belongs to the field of animal health and in particular Bovine Viral Diarrhea Virus (BVDV). The invention provides infectious BVDV clones and methods to produce said BVDV clones. The invention further relates to methods of attenuating said clones, attenuated BVDV clones and vaccines comprising said attenuated clones.
Bovine Viral Diarrhea Virus (BVDV) is the causative agent of BVD and mucosal disease in cattle (Baker, J. C., 1987, J. Am. Vet. Med. Assoc. 190:1449-1458; Moennig, V. and Plagemann, J., 1992; Adv. Virus Res. 41:53-91; Thiel, H. J. et al., 1996, Fields Virology 1059-1073). Fetal infection during pregnancy can result in the resorption of the fetus, abortions, as well as birth of immunotolerant calves which are persistently infected with BVDV. These calves lack or have very low neutralizing antibody titers and are continuously shedding high amounts of virus. Next to acutely infected cattle these calves are the major source for virus spreading and are therefore of prime importance in the epidemiology of this disease. The major economical impact of BVD results from high abortion rates, stillbirths, fetal resorption, mummification, congenital malformations, and birth of weak and undersized calves. For a detailed review of the pathogenesis, hereby refer to the article of Moennig, V. and Liess, B. of 1995, Virus, 11(3):477-487.
Two major antigenic groups of BVDV (type 1 and 2) have been described (Becher, P. et al. 1999, Virology 262:64-71) which display limited cross neutralizing antibody reactions (Ridpath, J. F., et al. 1994, Virology 205:66-74).
Present vaccines for the prevention and treatment of BVDV infections still have drawbacks (Oirschot, J. T., et al. 1999, Veterinary Microbiology, 64:169-183). Vaccines against the classical BVDV type 1 provide only partial protection from type 2 infection, and vaccinated dams may produce calves that are persistently infected with virulent BVDV type 2 (Bolin, S. R., et al., 1991, Am. J. Vet. Res. 52:1033-1037; Ridpath, J. F., et al., 1994, Virology 205:66-74). This problem is probably due to the great antigenic diversity between type 1 and type 2 strains which is most pronounced in the glycoprotein E2, the major antigen (Tijssen, P., et al., 1996, Virology 217:356-361). most monoclonal antibodies against type 1 strains fail to bind to type 2 viruses (Ridpath, J. F., et al., 1994, Virology 205:66-74).
Killed vaccines (inactivated whole virus) or subunit vaccines (conventionally purified or heterologously expressed purified viral proteins) are most often inferior to live vaccines in their efficacy to produce a full protective immune response even in the presence of adjuvants.
Live BVDV vaccines, although attenuated, are most often associated with safety problems. As mentioned above, they cross the placenta of pregnant cows and lead to clinical manifestations in the fetus and/or the induction of persistently infected calves. Therefore, they cannot be applied to breeding herds that contain pregnant cows. Pregnant cows have to be kept separate from vaccinated cattle to protect fetuses and must not be vaccinated themselves. Furthermore, revertants of attenuated live BVDV pose a serious threat to cattle. For conventionally derived attenuated viruses wherein the attenuation is achieved by conventional multiple passaging, the molecular origin as well as the genetic stability of the attenuation remains unknown and reversion to the virulent wild-type is unpredictable.
Live vaccines with defined mutations as a basis for attenuation would overcome the disadvantages of the present generation of attenuated vaccines. A further advantage of said attenuating mutations lies in their defined molecular uniqueness which can be used as a distinctive label for the attenuated pestivirus to distinguish it from pestiviruses from the field.
In the art, BVDV of defined genetic identity which closely resemble wild-type viruses are hardly known, in particular not for type 2 BVDV. In the art, there was a long lasting need for methods to generate such BVDV. Therefore, the technical problem underlying this invention was to provide a BVDV, in particular a BVDV type 2, of defined genetic identity.